The invention relates to electrocardiographic monitoring systems, and more particularly to such systems which detect abnormal ECG waveforms.
In the field of cardiology, as practiced both within the hospital and upon ambulatory patients, it is common to continuously monitor the ECG signal of a patient for analysis. Normally it is impractical for a doctor or other trained personnel to continuously monitor the ECG waveform and, accordingly, systems have evolved for automatically monitoring and analyzing the waveform. In some instances, this monitoring equipment may operate on a real-time basis while ECG signals are being obtained from the patient. In yet other applications, particularly the field known as ambulatory or Holter-type monitoring, the patient's ECG signal may be recorded over a long interval and subsequently replayed, usually at an increased speed, for the requisite analysis.
The ECG waveform is normally comprised of a series of characteristic points conventionally designated by the letters P, Q, R, S and T. The Q, R and S portions of the wave when taken together are referred to as the QRS complex. In most heart-monitoring systems, there is provided means for determining, in accordance with a preconceived method or formula, that the QRS complex has occurred. Such means are commonly referred to as R-wave detectors, a representative example of which being disclosed in U.S. Pat. No. 3,590,811 issued July 6, 1971, to Harris for Electrocardiographic R-Wave Detector. However, it is often desirable to further differentiate between a QRS complex which corresponds to a normal heart action, and a QRS complex which corresponds to abnormal heart action.
One such abnormal heart action to which particular attention has been directed is that of ectopic beats. Such ectopic beats are characterized by departure from a "normal" interbeat interval and/or departure from a "normal" waveform morphology. It will be appreciated that a "normal" interbeat interval and/or waveform morphology for one patient may differ from that which is "normal" for another. Certain ectopic beats deserving of special attention include ventricular premature beats (VPB), atrial premature beats (APB), and successive groupings of such premature beats.
Therefore, it is important to be able to analyze the ECG wave in a manner capable of rapidly and accurately identifying particular types of ectopic beats and characterizing them as such. It is further important that the analysis be applicable to the playback of prerecorded ECG waveforms as well as those received on a real-time basis.
Typically, ventricular premature beats (VPB's) have been identified by monitoring the interval between successive QRS complexes and the width and/or area of such complexes, and signaling the occurrence of such VPB if those parameters differ by more than a predetermined amount from that which is "normal" for the particular patient. An example of these techniques is disclosed in some detail in U.S. Pat. No. 3,616,791 issued Nov. 2, 1971, to G. J. Harris for Electrocardiographic Morphology Recognition System. In that patent, certain QRS complexes are identified as VPB's if their width is greater than normal. That determination of width is made by measuring the area under the rectified ECG signal and comparing it with the average area. That area determination is made by integrating the rectified waveform. Before actually indicating that a "wide" QRS complex is actually a VPB, the interval between QRS complexes is normally required to exhibit a so-called compensatory pause. The QRS complex of a VPB will usually occur earlier than expected and the next succeeding QRS complex will occur after a longer than normal interval or compensatory pause. This combination of a "wide" QRS complex occurring earlier than usual and followed by a compensatory pause is generally a reliable indicator of a VPB.
It would, however, be even more preferable to identify a VPB based on width alone without additionally requiring the analysis of the interbeat interval. However, such analysis has not heretofore been relied on, in part because the area, and thus the width measure, of the QRS complex may increase as the height or amplitude of the waveform increases due to changes in patient respiration, drift in the signal baseline and/or other causes.
Accordingly, it is a principal object of the present invention to provide an apparatus for analyzing the ECG wave in a manner which simply and accurately identifies VPB's. It is a further object of the invention to provide improved means for accurately identifying the occurrence of the QRS complex in an ECG waveform whereby accurate identification of VPB's and other heart characteristics may be made.